1. Field of the Invention
The present invention relates to a sensor for measuring a distance and a method for measuring the distance using the sensor, and more particularly to a sensor for measuring a distance using a high-frequency signal and a method for measuring the distance using the sensor.
2. Description of the Related Art
There are a variety of conventional methods for measuring a distance using a radar sensor. According to the types of transmitting/receiving signal, the conventional methods are classified into a method for employing a Continuous Wave (CW) signal, a method for employing a pulse, and a method for employing an impulse represented by an Ultra Wideband (UWB).
The distance measurement sensor for employing the CW signal transmits/receives continuous high-frequency signals, detects a phase difference of two high-frequency signals, such that it measures the distance using the detected phase difference. The above-mentioned distance measurement method based on the CW signal can manufacture a distance measurement sensor having a very low measurement error using a Frequency Modulated Continuous Wave (FMCW) method and a Frequency Stepped Continuous Wave (FSCW) method. However, the above-mentioned distance measurement method transmits or receives a high-frequency signal using the CW signal, such that the transmission signal and the reception signal are always transmitted at the same time.
If the transmission and the reception of the signal are executed over a single antenna, the above-mentioned method requires an isolator and a circulator for separating the transmission signal from the reception signal.
FIG. 1 is a schematic diagram illustrating a conventional radar transmission/reception end using a single antenna.
Referring to FIG. 1, the radar transmission/reception end includes a transmitter 103, an antenna 105, a receiver 104, and a signal separator 101.
The transmitter 103 is designed to transmit a high-frequency signal, and includes a power amplifier 1031, a coupler 1032, and a frequency generator 1033.
The signal separator 101 is designed to separate the transmission signal from the reception signal. For example, the circulator or isolator may be used as the signal separator 101. Due to characteristics of the transmission signal, some parts of the transmission signal leak to the reception signal. The reference number 102 denotes the leaking reception signal.
The receiver 104 is designed to receive the high-frequency signal, and includes a low-noise amplifier 1041, a mixer 1042 for comparing the high-frequency signal with a reference signal, and a low-pass filter (LPF) 1043 for removing the high-frequency signal. The output signal of the coupler 1032 contained in the transmitter 103 is used as the reference signal. However, generally, the isolator and the circulator are manufactured in the form of a large-sized product, such that it is impossible to integrate the isolator and the circulator, and it is difficult to separate the isolator and the circulator from each other.
In the meantime, the reason why the transmission end and the reception end use different antennas instead of a single antenna is to prevent leakage signals directly generated from the transmission/reception ends from being connected, such that the negative influence of the leakage signal can be greatly reduced.
Referring to FIG. 2, the radar transmission/reception end includes a transmitter 201 and a receiver 202.
The transmitter 201 is designed to transmit a high-frequency signal, includes a frequency generator 2014, a coupler 2013, a power amplifier 2012, and a transmission antenna 2011.
The receiver 202 is designed to receive the high-frequency signal, and includes a low-noise amplifier 2022, a mixer 2023 for comparing the high-frequency signal with the reference signal, a low pass filter (LPF) 2024 for removing the high-frequency signal, and a reception antenna 2021.
The above-mentioned conventional radar sensor employs a high-priced separation device for separating the transmission signal and the reception signal from each other, and transmits a high transmission power using an algorithm processed by a super computer and a large-sized antenna having superior directivity. Therefore, although the separation characteristics between the transmission end and the reception end are not greatly considered, there is no problem in operating the sensor.
However, in order to manufacture a very-small-sized radar sensor and apply the small-sized radar sensor to a variety of application fields, the conventional radar method is of no use.
Specifically, in the case of using two antennas, each of which determines its own size according to the frequency, a total size of the sensor increases, such that it is difficult to manufacture a very-small-sized sensor.
Also, the above-mentioned two antennas are connected to the integrated sensor, such that the first antenna and the second antenna are adjacent to each other, resulting in the coupling effect of the two antennas. Therefore, the leakage effect of the transmission signal occurs, such that it has a negative influence upon the reception end.
If the distance to be measured becomes longer such that the leakage signal is higher than the actually-received signal, the above-mentioned leakage signal encounters serious problems.
If the gain of the reception end increases to amplify a low signal, the leakage signal received from the transmission end is also amplified, such that it allows a signal processing end to be saturated, and it is impossible to receive the signal. Therefore, the actual distance capable of being actually measured is designed to be shorter than a maximum measurement distance determined by the sensor's specification, and the error of the actual measurement distance increases within the distance capable of being measured.
In this way, the negative influence caused by the transmission leakage signal in the very small-sized radar sensor based on the CW signal is inevitable.
Specifically, in the case of a direct-conversion radar sensor for including necessary information in a DC (Direct Current) signal, information caused by a DC offset encountered by the transmission leakage signal is distorted, resulting in a more serious problem.
The conventional radar sensor introduces a complicated algorithm to its reception signal, such that it reduces the leakage problem using a high-performance CPU (Central Processing Unit), or it minimizes the negative influence of the above-mentioned problem using an additional circuit. However, indeed, due to the limitation in the implementation principle, it is difficult to completely solve the above-mentioned problem.
Compared with the CW-based sensor, the influence of the transmission leakage signal is negligible in other sensors (i.e., the pulse-based sensor or the impulse-based sensor) because the pulse and the impulse are separated in time.
The pulse-based sensor and the impulse-based sensor are designed to measure the time from the transmission point to the reception point of the signal. According to the pulse-based sensor and the impulse-based sensor, the transmission end does not transmit the signal during the reception time, such that there is no transmission leakage caused by the transmission signal. However, if there is an overlapping part between the transmission signal and the reception signal, the sensor for recognizing the distance on the basis of a difference in time is unable to recognize the distance.
In other words, if the transmission signal and the reception signal are simultaneously received because the distance to be measured is short, the pulse-based sensor the impulse-based sensor cannot acquire the distance information, such that a minimum measurement distance must be limited.
Generally, the sensor for measuring the time using the pulse has difficulty in measuring the distance of less than 20 cm. In order to increase the accuracy and precision of the measurement, the pulse-based sensor and the impulse-based sensor must use a short pulse-width signal (i.e., a wide bandwidth signal) to the signal transmission/reception, such that the configuration of an overall sensor is complicated and it is difficult to design each circuit. Also, since the range of reducing the pulse width is limited, the measurement accuracy is lower than that of the CW-based sensor.
In the meantime, the 6-port structure using 6 ports has been proposed to substitute a Vector Network Analyzer (VNA) for precisely measuring a reflection coefficient in the 1970's.
FIG. 3 is a circuit diagram illustrating a 6-port structure denoted by passive elements.
Referring to FIG. 3, the 6-port circuit includes 2 input ports (a1 and a2) and 4 output ports (b1, b2, b3, and b4). The 4 output ports are represented by a linear relationship between a single reference signal (L0) received from the two input ports and another RF (Radio Frequency) signal.
The high-frequency signals of the four output ports (b1, b2, b3, and b4) can be converted into voltage signals via the power detector (See FIG. 6) and the LPF. If a mathematical calculation is applied to the voltage conversion using 4 output voltages, a reflection coefficient of the RF signal can be recognized.
The reference number 301 of FIG. 3 is indicative of a directional coupler for changing the phase by 90 or 180 degrees. The reference number 302 is indicative of a transmission line delayed by 90 degrees such that it can transmit the same phase signal in two ways. The reference number 303 is indicative of a resistor for removing the high-frequency signal. The above-mentioned 6-port circuit and a method for using the same have been disclosed in the U.S. Pat. No. 4,104,583, so that a detailed description thereof will herein be omitted for the convenience of description.
The above-mentioned conventional distance measurement sensor based on the 6-port structure must apply the high-frequency signal to two input ports at the same time, such that only the method for measuring the distance using the CW signal can be made available.
In conclusion, an improved remote-distance measurement sensor and an improved distance measurement method must be developed such that they can maintain the advantages of the CW-based measurement method without any change, and can solve the problems of the CW-based measurement method.